This invention relates to a frictionally engaging device for power transmission of an automotive vehicle, and more particularly to a frictionally engaging device of this kind which is applied to a transmission of a multi-stage type with a plurality of gear ratios or a transmission of an infinitely variable speed type having a fluid coupling such as a torque converter as well as a creep-inhibiting device, and which has a function of cooling the frictionally engaging elements thereof.
Creep-inhibiting devices, which are generally known, are classified into two types, one of which is adapted to bring the transmission into a neutral position to interrupt the power transmission between the engine and the driving wheels of the vehicle, upon detection of idling operation of the engine, while the other type is adapted to actuate the braking system or to establish a high-speed gear train and a low-speed gear train at the same time so as to generate to braking force by the so-called internal lock of the transmission upon detection of idling operation of the engine. It is generally considered that the former type is more advantageous in respect of fuel consumption and is less dangerous when it is misoperated.
However, according to the former type, in starting the vehicle, it is required to release the transmission from the neutral position so as to permit the frictionally engaging device, e.g. the starting or first-speed clutch, to promptly operate without delay. In order to ensure good responsiveness of the starting clutch to the starting operation of the vehicle, it is known to be effective to increase and hold the operating pressure of the starting clutch to and at a value which does not cause substantial engagement of the clutch. That is, the clutch operating pressure should be previously increased to and held at a pressure (hereinafter referred to as "the holding pressure Po") slightly lower than the minimum pressure (hereinafter referred to as "the engaging pressure Pe") at and above which the clutch becomes engaged, so as to bring the starting clutch from a creep-inhibiting state to a creep-permitting state with good responsiveness.
The holding pressure Po should desirably be set at a value as close as possible to the engaging pressure Pe. However, in practice, if the holding pressure Po is set at such a value very close to the engaging pressure Pe, the frictionally engaging elements of the clutch such as clutch facings can be forcibly held in sliding contact with each other while the clutch is in a creep-inhibiting state, due to machining errors and assembling errors of the clutch. If the clutch is kept in a creep-inhibiting state for a long period of time, the frictionally contacting clutch plates will generate heat, resulting in early waar of the clutch plates.